As illustrated in FIG. 1, the differential signal DPIN−DNIN is a cyclical switching signal in which DPIN and DNIN alternate in level during successive cycles and cross over during a switching interval 11 between the cycles. During each cycle, there is a stable period 12 in which both DPIN and DNIN remain constant in level. The differential signal is considered to be valid when it is greater in level than a reference signal VREF, i.e. when |DPIN−DNIN|>VREF, and invalid when it is below the level of the reference signal, i.e. when |DPIN−DNIN|<VREF.
In a typical application, the differential signal might, for example, have a level of about 150–180 mV, and the reference voltage might have a level on the order of 125 mV. The differential signal is a high speed signal, with a typical rate of on the order of 0.5 gigabit per second and a relatively wide common mode voltage VCOM ranging from about −50 mV to about +500 mV. The reference voltage and the common mode voltage are both referenced to ground. For the envelope detector to begin indicating the validity of the differential signal within a few cycles of start-up, it must complete its evaluation and indicate the validity of the signal within about 8 nanoseconds, or less. In this particular example, the differential signal is considered to be valid if DPIN−DNIN>VREF or DNIN−DPIN>VREF for about 0.5 nanosecond or more during the stable period.
Differential signals of this type are commonly found in and around personal computers, e.g. on USB keyboard cables.
Envelope detectors heretofore provided have had certain limitations and disadvantages such as only working with single-ended signals, relatively high signal levels (e.g., 0.7 volt), and relatively slow signals (e.g., 10 Mhz or less).